Cathode ray deflecting apparatus



@ 29 A A Hg. E 'A Oct. 14, 1941.

G. R. TINGLEY CATHODE RAY DEFLECTING APPARATUS Filed May 18, 1959 Fig. :1

Patented Oct. 14, 1941 George Richard Tingley, London, England, as-Y signor, by mesne assignments, to Cinema-Television Limited, Londo poration n, England, a British cor- Application May 18, 1939, SerialNo. 274,353

In Great Britain August 8, 1938 i claim. (i. o-157) This invention relates to denecting means for.

use with cathode ray tubes and similar apparatus of the type in which an electron beam is required to be deflected over a surface which is inclined to the undeflected direction of thetelectron beam and the present invention seeks to provide an improved form of electromagnetic deecting means for apparatus of this type.

When it is desired to cause an electron beam to scan a rectangular area on a surface which is inclined to the direction of the undeected elec? tron beam it is necessary to compensate for what is known as the keystone effect which is produced if the electron beam is caused to describe a rectangular scan at its point of deflection.

According to the present invention there is provided cathode ray tube apparatus comprising in spatial succession an electron gun, a first de' ecting system for deflecting the electron beam in a rst coordinate direction, a second deflecting system for deilecting the electron beam in a second coordinate direction and a screen to be scanned by the electron beam, said screen being inclined to the axis of the gun in said second coordinate direction and in which the direction of deflection of the electron beam by said second deiiecting system varies in accordance with the position of the beam in said first coordinate direction, the arrangement being such that keystone distortion is avoided or reduced.

It will be apparent that the deflection of the beam by the said second deflecting system is variable in direction and that when reference is made, as in the preceding paragraph, to a second coordinate direction of deection, it is to be understood that this refers to the mean position'of the beam in the said first coordinate direction.

According to a feature of the present invention said second deecting system comprises an electromagnetic yoke such that the lines of force traversed by the beam in the plane of the yoke lie substantially along concentric circles whose centre lies in the plane of the yoke and on that side of the undefiected position of the electron beam where the angle between said undeected beam and said screen is obtuse.

According to a further feature of the present invention said second deiiecting system comprises means for producing an electrostatic field such that the equipotential lines of the field lie substantially along concentric circles whose centre lies in the plane of the yoke and on that side of the undeflected position of the electron beam where the angle between said undeflected beam is obtuse.

Preferably the plane of the field corresponding to said second deflecting system is approximately parallel to the screen, the Vexact position being determined by experiment so as to obtain the best f results.

"According to a further feature of the present invention in addition to the apparatus as above described there is provided deiiecting means supplementary to said second deilecting means and l producing a constant deflectingfield for the purpose of producing a shift of the'scanned area on the screen. It has been found that by suitably disposing and arranging this supplementary deflecting means the amplitude of deiiection of the l electron beam produced by said second deecting means can be increased. Y

By the use of the present invention it is possible to produce a rectangular scan on a surface inclined at a considerable angle to the direction of the undeected electron beam and it is moreover found unnecessary to Vary the amplitude of deflection in the direction perpendicular to that produced by the reflecting means according to the invention. Normally a deflecting yoke according to the invention will be placed parallel with the scanned member but in certain cases where the degree of keystone correction is found to be insufficient it may be found advantageous to incline the yoke so that the ends of the extensions are nearer to the' plane of the scanned member than thelyoke proper.

In order that the nature of the operation of the present invention may be mo-re particularly described lreference is now made to the accompanying diagrammatic drawing, of which:

Figure l is a diagram illustrating the principle of the present invention;

Figure 2 is a side view; and

Figure 3 is a partial end elevation of deecting means according to the invention and a cathode ray tube with which the deflecting means is associated;

Figure 4 shows another form of deflecting means according to the invention;

Figure 5 illustrates an embodiment of cathode ray tube apparatus according to a feature of the present invention; and

Figures 6 and 7 illustrate alternative embodi-n ments of part of the apparatus shown in Figure 5.

In Figure 1, I represents the neck of a cathode ray tube or like electron discharge device disposed in the field produced between pole-pieces 2 and 3. This field is of such a nature that the lines of force represented by the dotted lines 4 are con- 'centric circles. Consider the deflection of an electron beam initially at the point which is on the axis of symmetry of the deflectng field produced by varying the strength of the field, the electron beam will be deflected perpendicular to the lines of force in the direction indicated by the arrow 6. If the deection of an electron beam initially at the point I be considered it will be seen that to travel normally to the lines of force of theV deflecting eld the beam must move in the direction of the arrow B. Similarly an electron beam initially at the point 9 will be deected in the v direction of the arrow Ill. Thus an electron beam which at the top of the scan is deflected hori-r zontally from 'I to 9 will at the bottom ofi the scan be deflected from 8 to I0 so that whenV the deiiecting yoke 2, 3 is placed substantially parallel with the scanned member and with that end at which the limbs are nearer together towards that part of the scanned member which is further from the electron gun from which the beam is produced, correction for the keystone distortion, which is introduced by the inclined position of the scanned member, is obtained if the radius of curvature of the lines 4 is appropriately small.

In Figure 2 is shown a cathode ray tube II in which an electron beam produced by a gun I2 is deflected in a vertical direction over a screen I3 by means of a yoke I4 in accordance with the present invention. As is shown in Figure 3 the yoke I4 comprises a U-shaped portion I5 upon which are wound the deiiecting lcoils I6 and which is provided with extensions I'I which are netic elds produced by deflecting coils 4, which may be surrounded by an iron member 5, and by a yoke 6 which may be of the type described with reference to Figures 1 to 4. By providing the yoke 'I with arms partially embracing the neck of the cathode ray tube as shown in Figure 6 or '7, an increased degree of scan amplitude correction may be obtained. The exact form and position of the yoke 'I are best determined by experiment for the particular design of 'cathode ray tube employed.

'Ihe form shown in Figure 'l has the advantage that it produces less focus distortion in cases where magnetic focussing is employed. If the gun voltage of'the cathode ray tube is increased the deflection sensitivity of the tube is reduced. To overcome this by the use of the present invention the current through the energising coil on the additional deflecting means is increased. By suitable design of the apparatus it may be arranged that the increases of deflection sensitivity inclined towards each other and which correcordance with the invention. Small coils 22 are placed on the extensions 2I for the purpose of compensating for any unbalanced D. C. component produced by the coils 20.

Alternatively, in accordance with a feature of the present invention, the effect of the unbalanced D, C. component may be overcome by the provision of additional means producing a constant deecting field. Such additional deflecting means is illustrated in Figures 5, 6 and '7'.

'Figure 5 shows a cathode ray tube I in which an electron beam produced by a gun 2 is deflectedv over a fluorescent screen 3 by appropriate magin each direction are equal and that the image shift'` produced by the additional deflecting means is such as to restore the scanned area to the position occupied before the increase in gun potential. Compensation for reduced gun voltage may be similarly arranged.

In certain cases it may be found advantageous to incline the yoke I 4 so that its top end is nearer to the screen l-3 than its lower end so that more lines of force pass through the region of deflection of the electron beam. The extensions I'I may be provided with comparatively short additional extensions hinged to their top ends in order to produce an additional concentration of the magnetic field in that region and thus decrease the radius of curvature of the lines of force. The yoke is preferably constructed of iron laminae of suitablel thickness interleaved at the joints. The deflecting coils may comprise the windings of a feed-back transformer as described in British patent specification No, 463,972.

I claim:

In. a cathode ray tube apparatus, an electron gun for producing an electron beam, a surface scanned by said beam, two deflecting systems for deecting the electron beam in coordinate directions, and means comprised in one of said systems for producing in the path of the beam electric lines of force forming arcs of substantially concentric circles intersecting the beam in substantially the same plane, said means comprising a U-shaped yoke having a coil on its central portion and provided with extensions at the ends of its limbs which extensions are inclined towards each other and are of substantially greater length than the central portion of the U.

GEORGE RICHARD TINGLEY. 

